In chapter II of Utilitarianism, John Stuart Mill states:

Utility, or the Greatest Happiness Principle, holds that actions are right in proportion as they tend to promote happiness, wrong as they tend to produce the reverse of happiness. By happiness is intended pleasure, and the absence of pain; by unhappiness, pain, and the privation of pleasure.

Pleasure and the absence of pain are therefore the only things desirable as ends. This is a simple enough thesis and it is arguable that the main reason for Utilitarianism being a book rather than a very short essay is the sheer number of objections its detractors have raised. Many of these are linguistic rather than strictly philosophical. The word "pleasure" tends to suggest sensual gratification, which Mill's critics describe as "swinish." Mill counters this objection rather neatly by saying that we only regard it as degrading to compare the pleasure of a human to the pleasure of an animal because the appropriate pleasures for a human ought to be higher: including "pleasures of the intellect; of the feelings and imagination, and of the moral sentiments."

To say that something is desirable as an end means that it is desirable in itself, not as a means to gaining something else. If I say that I want to be powerful, you may well ask me what I will do with the power when I have it. If I want to be rich, you might ask me what I will buy with the money. If, however, I say that I want to be happy, no one will ask what I plan to do with all the happiness, because it is an end in itself. It is worth noting that much of the moral opprobrium of sermons and satires is conventionally directed at those who mistake means for ends. The miser, for instance, is a stock figure of comedy because he is foolish enough to regard money as an end in itself.